Detectors and Analog Electronics

Cosmic RAy Telescope for the Effects of Radiation

Detectors and Analog Electronics

Bill CrainThe Aerospace Corporation

[email protected]


Introduction• Design Overview• Requirements Flowdown• Detector Specification• Signals, Noise, and Processing• Board Descriptions• Interface Block Diagram• Power Consumption• Trade Studies• Summary


Detector Electronics Design Overview• Detector electronics comprised of two board designs

– Detector Boards in Telescope assembly– Analog Processing Board (APB) in E-box

• Heritage approach from Polar CEPPAD/IPS unchanged from proposal– Linear pulse processing system utilizing Amptek hybrids– Circuits re-designed for CRaTER requirements

• Functional requirements summary– Measure LET of high LET particles in thin detectors– Measure LET of low LET particles in thick detectors– Provide good resolution for TEP effects– No fast timing requirements– Robust to temperature drift and environments


Analog Signal Flow Diagram

• Single fixed gain, linear transfer function• All detector channels use same topology

– Differences in preamp input transistor, detector biasing, and gain settings are made to optimize thin and thick channels

UnipolarLinear Output

Leading-edgeTrigger

dv/dt &Pole-Zero

Cancellation

ShapingAmp

Amp

BaselineRestorer

Preamp

Low-levelTrigger

SiliconDetector

BiasNetwork

TestPulser

Buffer


Requirements FlowdownElectronics reqs. derived from Level 2 reqs. and detector sims.Electronics Req. Thin Det. Thick Det. Level 2 Req. Affectivity

Amplifier strings 3 3 TEP analysis System design

Stability / drift 0.1% (1 bit) 0.1% (1 bit) Resolution Preamp, bias network, baseline restorer

Max. Energy Deposit 1 GeV for Fe 100 MeV for H+ LET spectrum range

Preamp range, closed-loop stability

Low E Threshold 2 MeV 200 keV LET spectrum range

Gain, baseline restorer, shaping time

Noise (rms) < 500 keV (1/2bit)

< 50 keV (1/2bit) Resolution Preamp, shaping time, thermal

Max. Singles Rate 10 kHz 10 kHz ESP events Shaping time, preamp time constant

Output Linear Linear Resolution Output amplifier

Internal Calibration 1000:1 1000:1 Operations Test pulser


Detector Specification (1)• Micron Semiconductor Limited

– Lancing Sussex, UK

• 20 years experience in supplying detectors for space physics– CEPPAD, CRRES, WIND, CLUSTER, ACE, IMAGE, STEREO,

and more…

• Detector Type– Ion-implanted doping to form P+ junction on N-type silicon– Very stable technology– Advantages to science include good carrier lifetime, stable to

environmental conditions, and thin entrance windows


Detector Specification (2)• Circular detectors having active area of 9.6 cm2

• Two different detector thicknesses: thin and thick– note: state-of-the-art is 20um for thin and 2,000um for thick detectors

• Guard ring on P-side to improve surface uniformity• Very thin dead layers (windows) reduce energy loss, lower

series resistance, and reduce noise

Guard P+ Contact Guard

Active Volume(depletion region)

P+ window

N window

0.1 um

0.1 um

140 um thin;1,000 um thick

35 mm, diam.

N contact

E-field


Detector Specification (3)

• Detector drawings (Micron)

Guard ring

Al. contact grid reduces surface resistivity

Al. contact plane


Detector Specification (4)

• ISO9001 – Full traceability and serialization– Travelers maintained

• Verification and test prior to detector shipment– Random vibration test– Thermal cycling and thermal vacuum– Stability

• Test criteria– Leakage current– I-V characteristic– Alpha resolution / pulser noise measurement (final test)


Proton Energy Deposition Simulations Reference:M. Looper

Thin 150MeV incident E Thick 150MeV incident E

Thin 1000MeV incident E Thick 1000MeV incident E1 GeV Protons

1

10

100

1000

10000

0.01 0.1 1 10 100

Deposited Energy (MeV)

Even

ts

Detector 2Detector 4Detector 5

20,000 incident protonsThick detectors (1000 um)

1 GeV Protons

1

10

100

1000

10000

0.01 0.1 1 10 100


Even

ts


20,000 incident protonsThin detectors (140 um)

150 MeV Protons

1

10

100

1000

10000

0.01 0.1 1 10 100


Even

ts


20,000 incident protonsThick detectors (1000 um)

150 MeV Protons

1

10

100

1000

10000

0.01 0.1 1 10 100


Even

ts


20,000 incident protonsThin detectors (140 um)

Nom

inal

Thr

esho

ld

Nom

inal

Thr

esho

ld

GEANT4


Iron Energy Deposition Simulation

Reference:J.B. Blake


Signal Characteristics

Detector Min Signal Max Signal Collection Time

DetectorCapacitance

FeedbackCapacitance

Thin 555E3 e-h pairs(88 fC)

275E6 e-h pairs(44 pC)

3 nsec e-drift9 nsec h-drift

700 pF 10 pF

Thick 55E3 e-h pairs(8.8 fC)

27.5E6 e-h pairs(4.4 pC)

38 nsec e-drift115 nsec h-drift

100 pF 1.0 pF

qμnNe(t)E qμpNh(t)E

PreAmp

Cdet

CFB

RFB

CFB (Ao) >> Cdet

AoVpk = Qtot/CFB


Signal Processing (1) • Combined dynamic range of thin/thick pair is 5,000 • Thin threshold to provide overlap with thick range• Thin Detector Signal

– Preamp input stage designed for 97% charge collection• High gain input jFET to raise dynamic input capacitance• 4% drift in operating point will result in 0.1% in output peak (< 1 bit)

– Large feedback capacitance needed to handle Fe deposit• Preamp compensation to maintain closed-loop stability

• Thick Detector Signal– Not as sensitive to detector capacitance– Design for low noise to maintain reliable 200 KeV low threshold

and achieve < 1-bit resolution


Noise Model (1)

Reference:Helmuth SpielerIFCA Instrumentation Course Notes2001

Detector Input Capacitance

Leakage Current

Shunt Resistance

Series Resistance

Preamp noise (ena)

Thin 700 pF Det.160 pF jFET+stray

200 nA @ 20C800 nA @ 35C

260 kohm 100 ohms 0.6 nV/√Hz

Thick 100 pF Det10 pF jFET+stray

200 nA @ 20C800 nA @ 35C

260 kohm 100 ohms 2.0 nV/√Hz

+ input cap.

T=peaking timeF=shaping factors


Noise Model (2)

Optimum


Signal Processing (2)

• Noise dominated by detector leakage and input jFET• Shaping time for both thin and thick detectors set at thick

optimum point– ~1 usec– Compatible with A/D signal acquisition timing– 3-pole gaussian shaping improves symmetry and 2-complex

poles shortens tail

• Shaping reduces noise but also impacts signal level – S/N at thick detector threshold for this design ~ 4– Translates to a noise occupancy in the coincidence window of <

0.1% for time period not greater than shaping time



• Other factors affecting noise performance– Bias resistor sized to minimize voltage drop (i.e., maintain

stable operating point)– Detector shot noise doubles every 8 C– Beneficial to operate cold; preferably below 20 C



• Pileup is rare due to low event rate and relatively short shaping time– Exception: occasional periods of high ESP flux

• Leading edge trigger technique causes timing uncertainty but coincidence window is large by comparison– Amplified low-level discriminator available to reduce walk

• Ballistic deficit is not an issue due to short collection times relative to peaking time of shaper

• Output voltage scaled for A/D input specifications


Detector Board

• Thin/thick detector pair use same design topology

• Signal collected on P-contact• Negative bias• Guard signal shunted to

ground• No guard leakage noise• AC coupling to isolate DC

detector leakage current• Low noise / high gain JFET

input stage

BiasNetwork

Test Input&

FeedbackNetwork

Negative Bias

Test Pulser

Feedback

Signal

P-contactG G

N-contact

Active Volume

JFET

BiasNetwork


Analog Processing Board

• Single board in E-box contains 3 thin and 3 thick detector processing channels

• Flight proven pulse processing components– Amptek A250 preamp hybrid utilizing external jFET on detector

board– Shaping amps use Amptek A275 for active filtering– Baseline restorer, Amptek BLR1, compensates for baseline shifts on

interface to A/D

• Pole-zero cancellation circuit for correcting preamp pulse decay to eliminate undershoot

• Test pulser injects ΔQ at preamp input jFET


Analog Interface Block Diagram

Bias

Temperature (uA/K)

Thin detector signalThin detector feedback

Thin test input

Returns

Detector Board 2(same)

Detector Board 3(same)

Power & Returns

Unipolar Signals

3 thin, 3 thickLow-level Triggers

Pulser Level (thick)Pulser Level (thin)

Det. Bias& Returns

Temperatures

MIT

1000 um

140 um

Thick detector signalThick detector feedback

Thick test input

Pulser Trigger

3 thin, 3 thick

Telescope Assembly(3 Detector Boards) Analog Processing Board

Aerospace

PHA System

Digital Processing


Power Estimate

Current Est. (mA) 10% Uncertainty Current Est. (mA) 10% Uncertainty24-May-05 24-May-05 24-May-05 24-May-05

+6V Analog 27.000 29.700 61.800 67.980 88.800 97.680

-6V Analog 0.000 0.000 41.800 45.980 41.800 45.980

+5V Digital 0.000 0.000 1.000 1.100 1.000 1.100

-100V Thin Det Bias 0.030 0.033 0.000 0.000 0.030 0.033

-300V Thick Det Bias 0.075 0.083 0.000 0.000 0.075 0.083

Power Est. (mW) 10% Uncertainty Power Est. (mW) 10% Uncertainty Total Est. (mW) Total Max. (mW)

Total Load 187.5 206.25 626.6 689.26 814.1 895.51

Total Est. (mA) Total Max. (mA)Analog Processing Board

Power SourceDetector Boards (total for 3 boards)

Total estimated power dissipation is < 1 Watt


Trade Studies

• Determine if one detector board can be implemented instead of three

• Determine if A250 device should be located on detector board

• Determine sensitivity of APB to detector performance– Want to avoid rework of APB when selecting/changing detectors

during development

• Determine how best to isolate chassis noise from detectors and preamp


Summary

• Detectors are well-established technology from experienced supplier

• Detector electronics design meets requirements of Level 2 mission and satisfies energy deposition levels determined by detector/TEP simulations

• Trade studies in progress

Documents

Detectors and Analog Electronics